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I'm going to be applying my 6th and last coat of Brightside tomorrow morning. I've used a 5% dilution w/ 333 for the last 3 coats (10% for the first 2). I've read that for best gloss the last coat should be undiluted. What is the downside to doing this? Are there more problems with lines? Is there anything I should know before doing this



Gee, 6 coats? I did my car in white with 4!
Lines--you mean lines from the roller edge? That's too much paint going on to the surface in one place. What I do to combat that is if I get a line, I go over the spot with the roller repeatedly applying very little pressure until the paint spreads out. The idea here is to not add any more paint to the surface, just work out what you've got on it.
I have diluted all of my coats about 10% and wound up with excellent gloss. I would be hesitant to not dilute at all for the final coat because I think the brushing liquid aids the paint in laying flat and hiding your brush strokes. I've gotten excellent results on most of my panels with few brush strokes and only minor orange peel. The gloss level is extremely high.